This invention relates generally to a technique and a system based thereon to automatically digitize engineering drawings and like documents and to store the digitized information in a computer accessible memory for subsequent retreival in order to reconstruct the drawing, and more particularly to a laser-beam scanning system adapted to digitize virtually all information contained in a line drawing, making it possible to thereafter faithfully reconstruct the scanned drawing.
The development in recent years of automatic drafting and computer-aided design has given rise to the concomitant need for the fast, accurate and flexible automatic digitizing of engineering drawings. To facilitate the transition between manual and computer-based systems, one must be able to convert existing hard copy drawings into digital form.
To this end, various types of raster scan systems have heretofore been proposed. One serious drawback of cathode-ray tube scanner-based systems is that, in general, they can function only with microfilm. As a consequence, all engineering drawings to be converted into digital form must first be micro-filmed. If microfilm versions of the drawings do not already exist, the need therefor can be costly and time-consuming.
Presently known laser or light beam scanner systems which make use of a raster scan permit the handling of large size drawings. But such systems are usually quite slow and require at least three minutes per square foot. Moreover, such raster systems scan the entire drawing without discrimination or editing, thereby generating enormous amounts of information. Thus for a typical D size drawing (2'.times.3'), about 16,000,000 bits of information are generated. While with careful encoding the number of bits can be reduced to about 1.6 million, even this is approximately three times the number of bits that present estimates indicate are essential to delineate a full drawing in point-to-point form.
In known systems, the raster scan output must be elaborately processed in order to determine which series of disconnected dots represent a line. Highly sophisticated "vectorizing " routines are required simply to decide which dot is connectd to which dot. This vectorizing process deteriorates the quality of the reconstructed drawing, as a result of which arcs, circles and curves are frequently recreated as jagged, rather than smooth segments. Furthermore, raster scanning systems, even those operating at three minutes per square foot, have limited resolution, and connection points are frequently missed. Hence the reconstructed drawing is deficient in many respects. For the typical engineering application, such deficiencies cannot be tolerated.
It is fair to say that the conventional and commonly practiced technique for scanning a drawing to extract raw information therefrom and to store this information while still in raw form in a computer memory, and to then manipulate the memorized information in order to vectorize and digitize it, is self-defeating. Inasmuch as the information on the drawing is already vectorized, the proper objective in digitization is to extract the information from a drawing without destroying its inherent vectorization.
Line following systems have been developed to eliminate the problem of vectorization and to obviate the generation of massive amounts of "empty" or valueless digital data. But existing line following techniques are characterized by line-following rates measured in just a few inches per second.